All cultivated forms of lettuce belong to the highly polymorphic species, Lactuca sativa, which is grown for its edible head and leaves. As a crop, lettuce is grown commercially wherever environmental conditions permit the production of an economically viable yield.
Lactuca sativa is in the Cichoreae tribe of the Asteraceae (Compositae) family. Lettuce is related to chicory, sunflower, aster, scorzonera, dandelion, artichoke and chrysanthemum. Sativa is one of about 300 species in the genus Lactuca. 
Lettuce cultivars are susceptible to a number of pests and diseases such as downy mildew (Bremia lactucae). This disease result in millions of dollars of lost lettuce crop throughout the world every year. Downy mildew (Bremia lactucae) is highly destructive for lettuce grown at relatively low temperature and high humidity. Downy mildew is caused by a fungus, Bremia lactucae, of which various strains or races exist.
Downy mildew causes pale, angular, yellow areas bounded by veins on the upper leaf surfaces. Sporulation occurs on the opposite surface of the leaves. The lesions eventually turn brown, and they may enlarge and coalesce. These symptoms typically occur first on the lower leaves of the lettuce, but under ideal conditions may move into the upper leaves of the head. When the fungus progresses to this degree, the head cannot be harvested. Less severe damage requires the removal of more leaves than usual, especially when the lettuce reaches its final destination.
Lettuce mosaic virus (LMV) mainly infects lettuce seeds, which is the primary way that the virus is introduced to lettuce in the fields, but also may infect numerous crops and weeds, thereby creating reservoirs of the virus. LMV also may be vectored by aphids, which spread the virus within a lettuce field and introduce it into lettuce fields from infected weeds and crops outside the field.
Symptoms of lettuce mosaic virus vary greatly. Leaves of plants that are infected at a young stage are stunted, deformed, and (in some varieties) show a mosaic or mottling pattern. Such plants rarely grow to full size; head lettuce varieties infected early fail to form heads. Plants that are infected later in the growth cycle show a different set of symptoms. These plants may reach full size, but the older outer leaves turn yellow, twisted, and otherwise are deformed. On head lettuce, the wrapper leaves often will curve back away from the head and developing heads may be deformed. In some cases brown, necrotic flecks occur on the wrapper leaves.
Of the various species of aphids that feed on lettuce, the currant-lettuce aphid (Nasonovia ribisnigri) is the most destructive species because it feeds both on the leaves of the lettuce as well as deep in the heart of the lettuce, making it difficult to control with conventional insecticides. The lettuce aphid feeds by sucking sap from the lettuce leaves. Although direct damage to the lettuce may be limited, its infestation has serious consequences because the presence of aphids makes lettuce unacceptable to consumers.
Although several known lettuce cultivars exhibit resistance against disease, irrespective of lettuce type, many lettuce cultivars affected produce large leaves that, when cut to smaller size pieces generally result in a lot of cut surface resulting in a diminished shelf life with respect to wound-induced discoloration of these cut surfaces. Light green or ‘blond’ varieties are considered better tasting by consumers, due to their association with leaves from the heart of a lettuce plant, which are milder tasting as well as paler colored than outer leaves.
Although several known lettuce cultivars may be harvested mechanically at young plant, i.e. babyleaf stage, no pest and disease resistant lettuce cultivars exist that may be harvested mechanically at mature stage and still provide leaf pieces that are of small, directly edible size and have an attractive blond color. Mechanical harvesting saves labor cost and improves labor conditions in comparison with commonly applied hand-harvesting methods.
There exists a need, therefore, for an mechanically harvestable lettuce variety which exhibits a combination of resistance against downy mildew (Bremia lactucae) strains BL:1-28 and strain FR10.021, currant-lettuce aphid (Nasonovia ribisnigri) and lettuce mosaic virus (LMV).
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.